ABSTRACT Dengue (DENV) and Zika (ZIKV) viruses are mosquito-borne viruses that are major medical and public health problems worldwide. DENV causes the most prevalent mosquito-borne viral disease of humans, and severe cases manifesting vascular leakage can be fatal. The related Zika virus (ZIKV) recently caused explosive epidemics across the Americas and has been associated with congenital birth defects and Guillain-Barr syndrome. Despite their substantial worldwide morbidity and mortality, no therapeutic agents exist for treatment of dengue or Zika. Nonstructural protein 1 (NS1) is a flaviviral protein that participates in viral RNA replication and in its secreted form plays important roles in host immune evasion and viral pathogenesis. We and others recently described novel roles for NS1 in directly triggering endothelial barrier dysfunction and inducing inflammatory cytokine production from human immune cells, contributing to vascular leak in vivo. In this proposal, we will evaluate the in vitro and in vivo efficacy of glycans against NS1-mediated pathogenesis, as well as against DENV and ZIKV infection in vivo. We have developed multiple methods to study DENV and ZIKV pathogenesis based on characterization of NS1-induced endothelial barrier dysfunction in vitro (e.g., hyper- permeability, disruption of the endothelial glycocalyx-like layer [EGL]) and in vivo, using murine models of virus- and NS1-induced disease with vascular leakage. Our preliminary results showing that a sulfated derivative of ?- glucan from Agaricus brasiliensis fungus (FR-S) has a protective effect in vitro and in vivo against DENV NS1- induced endothelial hyperpermeability are promising. We have also shown that FR-S has anti-DENV and anti- ZIKV activity in vitro. In collaboration with Dr. K. Godula (UC San Diego) we determined that specific synthetic GAG-mimetic molecules efficiently bind to flavivirus NS1. Given the contribution of NS1 to flavivirus pathogenesis and our preliminary results, we hypothesize that glycans inhibit NS1-induced EGL degradation and vas- cular leakage in vivo as well as viral infection and have potential as novel treatment modalities for dengue and Zika. The approach is innovative in that we target inhibition of severe disease manifestations, in addition to antiviral activity. Aim 1 will select the most promising inhibitor(s) of NS1-induced pathophysiological pathways of DENV and ZIKV based on prevention of endothelial dysfunction in vitro. We will screen 27 glycans, including FR and FR-S, 4 GAG-mimetics, sulodexide, and 20 cyclodextrin analogues (in collaboration with Dr. T. Sohajda, CycloLab), for their ability to prevent NS1-induced hyperpermeability. Aim 2 will investigate the in vitro mechanism of action of prevention of endothelial dysfunction by the selected compounds. Aim 3 will evaluate the therapeutic potential of the most active compounds against DENV and ZIKV NS1- and virus-induced vascular leak, morbidity, and mortality in vivo. Overall, this proposal address- es a critical need, identifying novel therapeutic strategies against two major flaviviral diseases, by developing glycan-based compounds that target both the virus and pathophysiological consequences of infection.